Currently, diesel engines are almost solely used as motors of ships in marine traffic. The fuel used in them is oil, which contains sulphur. As a result of combustion, sulphur oxides that are harmful to the environment are created. The current environmental protection regulations allow the usage of fuel containing a maximum of 4% sulphur in marine engines. The requirements for reducing the sulphur dioxide emissions of flue gases of ships will tighten in the near future.
One way to reduce the sulphur dioxide content of flue gases created in diesel engines is to use low-sulphur fuels. Their price when compared to the currently used high-sulphur fuels is, however, very high. In addition, raising the degree of processing of fuel increases sulphur dioxide emissions.
Another alternative is to clean the flue gases before emitting them. For this purpose, it has been proposed to install flue gas scrubbers in ships. One such scrubber is disclosed, for example, in publication WO 99/44722. The publication discloses a wet scrubber, where the flue gas from an engine is conveyed to a scrubber, where it is scrubbed with sea water. A problem of this solutions is that the salty sea water causes corrosion in the scrubber and flue gas ducts. The problems of corrosion can be solved by using corrosion-resistant materials, but their price is so high that the material expenses of the equipment and flue gas ducts will rise unreasonably high. In addition, the amount of scrubbing solution to be removed is large, because in order to reach a sufficient sulphur removal result, a large amount of scrubbing solution has to be used in the scrubbing.
It is also known to reduce the sulphur dioxide content of flue gases in power plants situated on land by scrubbing them with a NaOH-containing scrubbing solution. The scrubbing solution is formed by mixing NaOH with fresh water, i.e. water from a lake or a river, or with tap water purified of them, which water does not include salts, as sea water does.